Aerial lifts of the type including a bucket supported by a boom commonly include a hydraulic control handle supported by the bucket for use by an operator in a bucket for controlling a plurality of hydraulic control valves at the base of the boom. An example of such an aerial lift is illustrated in the Myers U.S. Pat. No. 2,946,196, issued July 26, 1960 and assigned to the assignee of the present invention. As illustrated there, it is common that the aerial lift will include three pilot operated main control valves for use in controlling operation of a pair of hydraulic cylinders provided to cause extension of the boom and a rotary actuator which functions to cause rotation of the articulated boom about the central vertical axis. The control handle in the bucket is connected by a plurality of hydraulic fluid lines to the three pilot operated main control valves. The control handle can be moved in three different directions, and means are provided such that movement of the control handle forward and back in one direction will control operation of one of the main control valves, movement of the control handle up and down will control a second one of the main control valves, and twisting movement of the control handle will cause operation of the third one of the main control valves. The control handle includes a plurality of hydraulic pistons which move in response to movement of the control handle. Movement of the control handle and consequent movement of the pistons functions to cause a change in the hydraulic fluid Pressure in the hydraulic fluid lines connected to the control valves.
One of the problems with such prior art arrangements is that any change in oil pressure at the pilot operated control valves requires the operator to manually move the control handle to manually pump hydraulic fluid through the hydraulic fluid lines. In the preferred case, the control handle should provide precise control over the boom with a minimal amount of physical effort by the operator. In those applications where the articulated boom is very long, the hydraulic fluid lines must also be very long, and there may be substantial frictional losses between the bucket and the valves at the base of the boom. Where there are such fluid pressure losses, precise control of the boom is more difficult because movement of the control handle requires greater effort by the operator. In such applications where the articulated boom is quite long, it is of even greater importance that the control mechanism provide precise control, since a small signal to the hydraulic cylinders or rotary actuator will produce a substantial movement of the operator's bucket.
In the prior art arrangements, the force the operator must exert on the control handle also increases in cold weather due to changes in the viscosity of the hydraulic fluid.
Efficient operation of the prior art hydraulic control arrangements also requires an absence of leaks in the fluid lines between the control handle at the end of the boom and the control valves at the base of the articulated boom. The hydraulic system functions because of the incompressability of the hydraulic fluid in the system. If there are leaks in the system, the hydraulic controls become less effective as a means for accurately controlling the position of the bucket. The fluid lines must remain full of hydraulic fluid so that movement of the pistons connected to the control handle to cause a change in hydraulic fluid pressure in the fluid pressure lines at the bucket will also cause a similar change in the hydraulic fluid pressure at the pilot operated control valve.